1. Field of the Invention
The present invention relates to an optical pickup apparatus configured to read out a signal recorded in an optical disc with a laser beam.
2. Description of the Related Art
There are widely used optical disc devices capable of reading out or recording a signal by applying a laser beam emitted from an optical pickup apparatus onto a signal recording layer provided in an optical disc.
While there is commonly used an optical disc device in which an optical disc called CD or DVD is used, there has recently been commercialized an optical disc device in which an optical disc with improved recording density, i.e. Blu-ray disc or HD DVD (High Density Digital Versatile Disc), is used. In order to improve the density of a signal recorded in an optical disc, the spot diameter of a laser beam applied to a signal surface of the optical disc should be decreased, and to that end, the wavelength of the laser beam should be decreased. An optical pickup apparatus used in the optical disc device includes a laser diode generating a laser beam.
An infrared laser beam is used for reproducing a signal recorded in a CD standard disc described above; a red laser beam is used for reproducing a signal recorded in a DVD standard disc; and a blue-purple laser beam is used for reproducing a signal recorded in a Blu-ray standard disc or an HD DVD standard disc (next generation DVD).
There is provided an idea of commercialization of an optical disc device in which optical discs of all the above standards can be used, which necessitates an optical pickup apparatus enabling a signal recording and reproducing operation for all the above optical discs. Such an optical pickup apparatus requires the individual laser beams corresponding to the optical disc standards, which results in the incorporation in one housing of, for example, three laser diodes that are: a laser diode emitting the laser beam for CD; a laser diode emitting the laser beam for DVD; and a laser diode emitting the laser beam for next-generation DVD. When supplied with a drive signal, the laser diode in an optical pickup apparatus performs the operation of generating and emitting a laser beam, which is accompanied by the heat generation. In an optical pickup apparatus including a number of laser diodes, not only the layout of optical system components becomes complicated but also the optical pickup apparatus itself is miniaturized, and for this reason, it is required to dissipate heat given off from a laser diode with efficiency.
There is commonly adopted a technique for such a heat dissipating operation utilizing a thin cover made of metals such as stainless or copper, which is provided for protecting the optical system components (See Japanese Patent Laid-Open No. 2005-332465).
The cover described in the Japanese Patent Laid-Open No. 2005-332465 is fixed to a frame with one screw by screwing and with fixing portion. The above fixing portion constituted by a bent strip which serves for positioning, and the cover is fixed to the frame only with a screw.
In such a structure, there is a problem that when an impact is given to the optical pickup apparatus, the cover thereof slips out of place due to an insufficient force for fixing the cover so that the optical characteristics of the optical pickup apparatus are badly affected. As a method for solving such a problem, a method to fix the cover to the frame by using more than two screws is practiced.
A frame or a cover is typically made of metal, however, the frame and the cover are different in metal material such as: aluminum, zinc, and magnesium for the frame; and copper and stainless for the cover. Therefore, when the optical pickup apparatus rises in temperature due to the heat given off from the laser diode, a bias force acts between the frame and the cover due to the difference in expansion or shrinkage rate between metals. Therefore, in the optical pickup apparatus with the cover thereof fixed to the frame with more than two screws, a phenomenon such as twist occurs in the frame or the cover, so that the optical system constituting the optical pickup apparatus is badly affected, which is a problem.